Water Problems — Manganese
Water Quality Concerns: The Presence of Manganese in Drinking Water
The Occurrence and Impact of Manganese
Manganese, a mineral naturally found in rocks and soil, can also be introduced into water supplies through underground pollution sources. Unlike iron, manganese is rarely found alone in water; it is commonly present in iron-bearing waters but less frequently than iron itself. Chemically, manganese is closely related to iron, sharing similar forms and properties. Its presence in water can be highly problematic, often more so than iron. Even in low concentrations, manganese can cause unsightly stains on any surface it touches.
Manganese Deposits and Staining Issues
When manganese accumulates in pipelines, it can lead to black sediment and turbidity in tap water due to precipitation. This mineral’s oxidation results in dark brown or black stains on fabrics washed in manganese-rich water. The U.S. Environmental Protection Agency (EPA) Secondary Drinking Water Regulations recommend a manganese limit of 0.05 mg/l to prevent such staining. For industrial purposes, the acceptable manganese content is even lower, typically ranging from 0.01 to 0.02 mg/l, and sometimes even this is considered excessive.
Water Quality and Aesthetic Concerns
At concentrations higher than 0.05 mg/l, manganese can alter the color, odor, or taste of water, making it noticeable. However, the EPA states that health effects are not a concern until concentrations reach approximately 10 times the recommended limit. If the presence of manganese in water is unbearable, various treatment technologies such as cation exchange water softening, distillation, filtration, and reverse osmosis can effectively remove it.
Health Implications of Manganese Exposure
Manganese can be ingested through diet and drinking water, but exposure through bathing and showering in manganese-rich water is not a concern as the mineral does not penetrate the skin or become airborne. High exposure to manganese has been linked to nervous system toxicity, producing symptoms similar to Parkinson’s disease. Manganese is not typically associated with other types of toxicity, such as cancer or reproductive damage. Young children tend to absorb more manganese and excrete less compared to older individuals, making clean drinking water especially crucial for pregnant women and children.
Removal of Manganese from Water
Due to its slower oxidation rate compared to iron, manganese is generally more challenging to remove from water. Pure elemental manganese is gray with a pink tinge, brittle, and harder than iron. In nature, pure manganese metal is not found, but this chemically active element is present in many compounds. Manganese deposits are found in certain parts of the United States and other global regions.
Chemical Properties of Manganese in Water
In water, manganese is most commonly found as a manganous ion (Mn++). Salts of manganese are more soluble in acidic water than in alkaline conditions, similar to iron. Manganous ions typically enter water through the solubility of manganous bicarbonate. Some surface waters and shallow wells also contain organic or colloidal manganese compounds. Manganese bacteria can cause issues similar to those caused by iron bacteria, including clogging and staining.
Addressing High Concentrations of Manganese
Suspended insoluble manganic hydroxide, known as "black water," is less common but can occur. This is likely due to the higher pH required to precipitate manganic hydroxide compared to ferric hydroxide. Manganous bicarbonate in solution is colorless, which means that deep well waters containing manganous ions appear clear when freshly drawn. However, exposure to air quickly converts the clear manganous ions into the black insoluble substance, manganese dioxide, leading to various issues.
Chemical Reactions and Oxidation of Manganese
The reactions involved in the oxidation of manganese are as follows:
- 2Mn++ + O2 + 2H2O → 2MnO2 + 4H+
Treatment Options for Manganese Removal
Light concentrations of manganese can be removed using a water softener. For higher concentrations, oxidizing filters can be effective. Very high concentrations or those complicated by organic matter require chemical oxidation, similar to iron, followed by filtration. Chlorine is not fully effective in oxidizing manganese unless the pH is above 9.5, whereas potassium permanganate is effective at pH values above 7.5, making it the preferred oxidizing agent in most cases.
Conclusion
Manganese, while a naturally occurring mineral, can pose significant challenges when present in drinking water. Its ability to cause stains, alter water quality, and potentially affect health makes it a critical concern for water treatment and management. Understanding the properties of manganese and the available treatment methods is essential for maintaining clean and safe water supplies.